Add the oil to a 2-quart casserole and place in the oven for 5 minutes.
In a medium bowl, beat the eggs, salt, pepper then add the tomatoes, spinach, and scallions, mix until combined and then add the crumbled feta cheese.
Remove the casserole dish from oven, pour in the egg mixture and bake until the frittata is brown around the edges and a knife comes out clean from the center. 20 – 30 minutes.